This invention is directed to siloxane MQ resin based polyethers, and to the formation of vesicles. More particularly, the invention is directed to vesicles formed from certain water dispersible siloxane MQ resins, and the entrapment of water-soluble and water-insoluble substances.
The usefulness of certain "linear" polydimethylsiloxane polyether copolymers for forming vesicles has been demonstrated in U.S. Pat. No. 5,364,633, issued Nov. 15, 1994, and assigned to the same assignee as the present application. According to the '633 patent, vesicles can be formed with linear organosilicon compounds with one of the formulas: ##STR1##
In the formulas, R is methyl, --(CH.sub.2).sub.x O(C.sub.2 H.sub.4 o).sub.y (C.sub.3 H.sub.6 O).sub.z R', or --(CH.sub.2).sub.x N.sup.+ R.sub.3 "A.sup.-, provided at least one R radical is not methyl. R' is hydrogen, methyl, or an acyl radical. R" is a C1 to C6 alkyl radical, phenyl, benzyl, or --CH.sub.2 CH.sub.2 OH. The counterion A.sup.- is chloride, bromide, iodide, cyanide, methyl sulfate, salicylate, or a dodecylsulfate radical. Representative of these linear compounds are R--Si(Me).sub.2 O!.sub.14 --Si(Me).sub.2 --R in which R is --(CH.sub.2).sub.3 (OCH.sub.2 CH.sub.2).sub.7 OH; R--Si(Me).sub.2 O!.sub.14 --Si(Me).sub.2 --R in which R is --(CH.sub.2).sub.3 (OCH.sub.2 CH.sub.2).sub.12 OH; Me.sub.3 SiOSi(Me).sub.2 O!.sub.22 --SiMeRO!.sub.2 --SiMe.sub.3 in which R is --(CH.sub.2).sub.3 (OCH.sub.2 CH.sub.2).sub.12 OH; and Me.sub.3 SiOSi(Me).sub.2 O!.sub.103 --SiMeRO!.sub.10 --SiMe.sub.3 in which R is --(CH.sub.2).sub.3 (OCH.sub.2 CH.sub.2).sub.12 OH.
The present invention, however, is an improvement over the '633 patent, in the provision of "cross-linked" siloxanes which are chemically more inert, and in the unexpected discovery that vesicles can be formed from cross-linked siloxanes which are not previously known to possess this unique capability. Thus, prior to this invention, it was not known to form vesicles from cross-linked siloxane molecules such as the MQ resin based polyethers described herein; nor was it known to use water dispersible cross-linked siloxane MQ resins for entrapping water-soluble and water-insoluble substances.
Because of the nature of the siloxane linkage, surface active siloxanes do not follow the usual rules of surfactant activity, with regard to such things as aggregate formation and solubilization. Therefore, to even find a siloxane molecule which is able to form vesicles is quite surprising and unexpected. What is even more surprising and unexpected is that having once pinpointed a particular siloxane molecule for vesicle formation, that the vesicle formed from the siloxane would also be useful to entrap substances.
The advantages and benefits to be derived by the use of surface active siloxanes in vesicle formation and substance entrapment, include the fact that siloxanes possess a non-hydrocarbon character, and therefore provide a different set of physical properties than is currently available with hydrocarbon-based surfactant molecules. Secondly, surface active siloxanes have been found to form vesicles "spontaneously" on contact with water, and therefore they eliminate the use of energy intensive processes such as sonification, which are required for non-siloxane based surfactants. Thirdly, because the siloxane backbone offers chemically reactive sites, it is possible to easily exploit the formation of polymerized vesicles.
It is known that aqueous dispersions of lipids in the form of particles having a lamellar structure termed "liposomes", are excellent vehicles for the delivery or encapsulation of pharmaceutical substances. As a cosmetic system, they facilitate the supply of lipids and water to the stratum corneum, and are able in the absence of a moisturizer, to prevent the occurrence of dry skin. As a vehicle, they also effectively facilitate the transport of diverse substances such as moisturizers, tanning agents, and sunscreens into the stratum corneum, and furthermore prevent subsequent elimination by water washing.
In that context, lipid means specifically a class of surface active lipids, for example, phospholipids or lecithins which are dispersible into water to form lamellar phase particles or liposomes. Liposomes are single or multi-layered, spherical, globular, or tubular vesicles, the membranes of which consist of a bilayer of amphiphilic lipid molecules. Most cosmetic and pharmaceutical liposomes are composed of various phospholipids of natural, semi-synthetic, and synthetic origin.
The term vesicle refers to a structure consisting of a closed bilayer membrane envelope. Vesicles are often globular or tubular in shape but can be quite irregular. In the context of the present invention, however, vesicles made using surface active siloxanes are not liposomes in the sense that they are not actually prepared from materials recognized as lipids. Rather, a liposome is a specific type of vesicle prepared from lipid amphiphiles.
Unsolved problems relating to large scale production, stabilization, and safety of liposomes, may hamper development of this type of technology. On the other hand, vesicles formed using the surface active siloxanes of the present invention offer a potential for overcoming these problems, because of the different physical properties and the distinctly non-lipid character of vesicles formed from such surface active siloxanes.